Collision Origin
High-energy particle interactions with solid-state targets begin when an incoming radiation quantum collides directly with a lattice nucleus. The primary knock-on atom is the first target atom displaced from its stable crystal site by the incident radiation. This atom carries the kinetic energy transferred during the primary collision and acts as the source of further displacement damage.
Energy Cascade
Displaced atoms with sufficient kinetic energy will travel through the lattice and collide with other stationary atoms. The primary knock-on atom generates a localized damage cascade that produces many secondary displacements. This cascade of events continues until the kinetic energy of the moving atoms falls below the threshold energy required for displacement.
The resulting volume of high defect density is often referred to as a displacement cascade or defect cluster. Such highly localized regions can contain hundreds of unstable point defects that either recombine or form stable complex defects.
Threshold Requirement
Creation of a displaced atom is only possible when the energy transferred from the incident particle exceeds a specific limit. If the primary knock-on atom receives less than the threshold displacement energy, the energy is dissipated as localized thermal vibrations or phonons. The threshold value varies with crystallographic direction, and it is usually verified using electron beam irradiation where the energy transfer can be precisely controlled.
Defect Contribution
The spatial distribution of the generated defects is determined by the energy of the first displaced atom. High-energy primary knock-on atoms create dense, disordered regions that are difficult to anneal. These clusters of vacancies and interstitials can severely degrade the noise performance of silicon imaging arrays and the switching speed of power transistors.
